1. Field of the Invention
The present invention relates to an interferometer for measuring wave front aberration, and in particular to such apparatus adapted for evaluating the performance of an optical head for a compact disk as a signal reading device, by measuring the status of wave front of a light beam emitted by such optical head at photoelectric information reading.
2. Description of the Prior Art
Among interferometers for measuring the state the wave front of a light beam emitted by an optical apparatus, there is already known a so-called Machzender interferometer of a structure as shown in FIG. 1. In FIG. 1 there are provided an optical apparatus to be measured, having a light source and an optical system therein and emitting a light beam; a collimating lens 3 for converting a light beam, emitted by said apparatus 1, into a flat wave; beam splitters 4, 5; mirrors 6, 7; a beam expander 8 for increasing the diameter of the incident beam; a glass plate 9 for compensating the difference in optical path lengths of two beams split by the beam splitter 4; a density filter 10 for balancing, by attenuation, the intensities of two beams split by the beam splitter 4; and an interference fringe observing system. In said interferometer, a light beam passing through the beam splitter 4 is expanded by the beam expander 8 to form a reference wave front. On the other hand, a light beam reflected by the beam splitter 4 is adjusted to have an intensity and an optical path length, by the density filter 10 and the glass plate 9, same as those of the reference wave front, thus forming a so-called object wave front. Said reference wave front and said object wave front are superposed by the beam splitter 5 to generate interference fringes, which are observed through the observing system 11. The state, or wave front aberration, of the light beam emitted by the apparatus 1 to be measured can be determined by the measurement of the state of thus generated interference fringes.
However such interferometer is associated with drawbacks of requiring a fine adjustment of the thickness of the glass plate 9 for matching the optical path lengths of the reference wave front 12 and the object wave front 13, and of poor adjustability and stability because changes in angles of the beam splitters 4, 5 and of the mirrors 6, 7 readily give rise to a change in the number of interference fringes. Such phenomena are encountered not only in the above-explained interferometer but also in various other interferometers.